A typical prior art communication system comprises a transmitting station and a receiving station, and a connecting medium called a channel. Two-way communication requires each station to have both a transmitter and a receiver. FIG. 1 is a functional block diagram of a prior art communication system. The transmitting subsystem 102 of this communication system 100 accepts either digital or analog signals as inputs. An analog-to-digital converter 104 is coupled to receive an analog input signal 106 and to periodically sample the analog input waveform. The digital signal 108, comprising discrete voltage levels, output from the analog-to-digital converter is coupled to be received by a source encoder 110. The general purpose of the source encoder 110 is to convert effectively each discrete symbol into a suitable digital representation, often binary.
In some systems where no channel encoding function 112 is present, the source encoder 110 output is converted directly to a suitable waveform within the modulation function for transmission over the channel. Noise and interference added to the waveform cause the receiver's demodulation operation to make errors in its effort to recover, or determine, the correct digital representation used in the transmitter. By including the channel encoding 112 function in the typical communication system, the effects of channel-caused errors can be reduced. The channel encoder 112 makes this reduction possible by adding controlled redundancy to the source encoder's 110 digital representation in a known manner such that errors may be reduced. The channel encoded signal is coupled to be received by the modulator 114. The modulator 114 converts the binary symbols of the source information into a suitable waveform for transmission over the channel 116 using a signal with a particular carrier frequency.
The functions performed in the receiving subsystem 118 typically reflect the inverse operations of those in the transmitting subsystem 102. The demodulator 120 recovers the best possible version of the output that was produced by the channel encoder 112 at the transmitter subsystem 102. The channel decoder 122 reconstructs, to the best extent possible, the output that was generated by the source encoder 110 at the transmitter subsystem 102. It is here that the controlled redundancy inserted by the channel encoder 112 may be used to identify and correct some channel-caused errors in the demodulator's 120 output. The source decoder 124 performs the exact inverse of the source encoding 110 function.
As previously discussed, the purpose of the channel encoder is to convert the source code to a form that will allow the receiver to reduce the number of errors that occur in its output due to channel noise. As such, the channel encoder adds redundancy to the source code by inserting extra code digits in a controlled manner so that the receiver can possibly detect and correct channel-caused errors. One class of encoding process uses a coding method and apparatus that produces convolutional codes.
Convolutional codes involve memory implemented in the form of binary shift registers having K cascaded registers, each with k stages. The sequence of source digits is shifted into and along the overall register, k bits at a time. Appropriate taps from the various register stages are connected to n modulo-2 adders. The output code becomes the sequence of n digits at the output of these adders generated once for every input shift of k source digits. The ratio k/n is called the code rate, and K is called the constraint length. Therefore, each n-bit output codeword depends on the most recent k source bits stored in the first k-stage shift register as well as K-1 earlier blocks of k source bits that are stored in the other registers.
Tree diagrams, trellis diagrams, and state diagrams may be used to describe a convolutional code. The number of branches in a tree diagram doubles each time a new input digit occurs. For a long sequence of input digits to be encoded, the usefulness of the tree diagram is limited. A better approach uses a trellis diagram because the trellis diagram, while carrying the same information as a tree diagram, makes use of the fact that the tree is periodic in the steady state condition and involves only a finite number of states. The typical convolutional encoder of rate k/n and constraint length K will have 2.sup.k branches leaving each state node making the number of possible states 2.sup.k(K-1).
In the receiver subsystem, the demodulator will estimate what sequence of binary digits is being received over the channel. The purpose of the channel decoder is to accept the erroneous sequence of demodulator output digits and produce the most accurate replica possible of the source sequence that was input to the channel encoder of the transmitter subsystem.
For convolutional codes, the optimum decoding process amounts to finding the single path through the code trellis that most nearly represents the demodulated bit sequence. The transmitted code digits correspond to a specific path through the trellis. However, the receiver has no knowledge of the exact path and it can only use the received sequence, which possibly has errors, to find the most likely path that corresponds to the received sequence. This most likely path is then used to specify the decoded data sequence that would have generated the path. This procedure is called maximum-likelihood decoding. The Viterbi algorithm is a maximum-likelihood decoding procedure based on finding the trellis path with the smallest distance between its digit sequence and the received sequence. Typically, the distance used is the Hamming distance wherein the Hamming distance between two codewords of the same length is defined as the number of digits that differ in the two sequences. For example, the sequence "011010111" differs from the sequence "111001101 in digits 1, 5, 6, and 8, so the Hamming distance is 4.
Over the last several years, the development and use of wireless communications has, been significant. In the 1980's, numerous analog cellular networks were implemented, many of which quickly reached capacity limits, especially in the large service areas of metropolitan cities. The wireless telecommunications industry, in anticipation of these limitations, introduced several digital technologies to increase spectral efficiency and enhance wireless communications. The enhancements included the addition of features and services such as facsimile and data transmission and various call handling features. Thus, wireless communication technology has evolved from simple first-generation analog systems for business applications to second-generation digital systems with features and services for residential and business environments. Currently, the third-generation systems are being developed, known as personal communications systems (PCS). These PCS systems will enable the wireless network to deliver telecommunication services, including voice, data, and video, without restrictions on the portable terminal, location in the world, point of access to the network, access technology, or transport methods.
In the digital technologies associated with wireless communications, there are two basic strategies whereby a fixed spectrum resource can be allocated to different users: narrowband channelized systems and wideband systems. Two narrowband systems are the frequency-division multiple access (FDMA) systems and the time-division multiple access (TDMA) systems. In terms of improved capacity, the wideband systems are the better alternative because the entire system bandwidth is made available to each user and is many times larger than the bandwidth required to transmit information. Such systems are referred to as spread spectrum systems.
Among the many multiple-access technologies available for cellular and PCS systems, the digital spread spectrum code-division multiple access (CDMA) technology has been adopted as a standard in North America. The CDMA system reuses the same frequency in all cells to increase the capacity. The CDMA, as used for digital cellular phone applications, comprises an uplink, or mobile to base station link, and a downlink, or base station to mobile link, each having a dedicated band of frequencies. The CDMA channels are defined in terms of a radio frequency (RF) and code sequence. Sixty-four Walsh functions are used to identify the downlink channels, whereas a long pseudo-random noise (PN) code with different time shifts is used to identify the uplink channels.
Code-division multiple access (CDMA) communication systems used by the current generation of wireless telephone networks typically use direct sequence spread spectrum signaling techniques because of the robustness of these systems to interference. The direct sequence spread spectrum system is a wideband system in which the entire bandwidth of the system is available to each user. A direct sequence spread spectrum system, also referred to as a pseudo-noise system, is characterized by a carrier that is modulated by a digital code in which the code bit rate is much larger than the information signal bit rate. Therefore, the bandwidth of the transmitted signal, s(t), is much greater than that of the message, m(t). The spreading of the data is performed by means of a spreading signal, called a code signal, that is independent of the data and is of a much higher rate than the data signal. This means that the spreading signal has a bandwidth much larger than the minimum bandwidth required to transmit the desired information, which for a digital system is the baseband data.
Furthermore, the relatively wide bandwidth of s(t) caused by the independent modulating waveform, of spreading signal c(t), means that the spreading signal must be known by the receiver in order for the message signal, m(t), to be detected. Therefore, despreading is accomplished at the receiver by the cross-correlation of the received spread signal with a synchronized replica of the same signal used to spread the data. Consequently, the complex envelope of the spread spectrum signal is a function of both m(t) and c(t). In the typical case, a product function is used, so that EQU g(t)=g.sub.m (t)g.sub.c (t) (1)
where g.sub.m (t) and g.sub.c (t) are types of modulation complex envelope functions.
The spread spectrum signals are classified by the type of mapping functions that are used for g.sub.c (t). With a direct sequence spread spectrum system, the information waveform, m(t), typically comes from a digital source so that m(t) is a polar waveform having values of .+-.1. Furthermore, binary phase shift keyed (BPSK) modulation has g.sub.m (t)=A.sub.c m(t). Thus, for direct sequence where g.sub.c (t)=c(t) is used in equation 1, the complex envelope for the spread spectrum signal becomes EQU g(t)=A.sub.c m(t)c(t) (2)
The resulting s(t)=Re{g(t).sup.j.omega.ct } is called a binary phase shift keyed data, direct sequence spreading, spread spectrum signal, and c(t) is a polar spreading signal. Moreover, this spreading waveform may be generated by a pseudo-random noise (PN) code generator where the values of c(t) are .+-.1. The PN code generator typically uses a modulo-2 adder and r clocked shift register stages.
FIG. 2 is a prior art BPSK direct sequence spread spectrum transmitter 200. The transmitter may comprise a source encoder (not shown) coupled to receive an input data sequence. The transmitter 200 comprises a BPSK modulator 202 that is coupled to receive a source encoded input data sequence. The BPSK modulator 202 generates a BPSK signal 204. A spreader 206 is coupled to receive the BPSK signal 204. The spreader 206 outputs a BPSK direct sequence spread spectrum signal 208.
FIG. 3 is a prior art BPSK direct sequence spread spectrum receiver 300. The receiver 300 comprises a despreader 304 that is coupled to receive a transmitted BPSK direct sequence spread spectrum signal along with channel noise 302. The output of the despreader 304 is coupled to a demodulator 306. The demodulator 306 is coupled to provide a demodulated BPSK signal 308 to a decoder (not shown).
Orthogonal functions are typically employed to improve the bandwidth efficiency of a spread spectrum CDMA system. Each mobile user uses one member of a set of orthogonal functions representing the set of symbols used for transmission. While there are many different sequences that can be used to generate an orthogonal set of functions, the Walsh and Hadamard sequences make useful sets for CDMA. Typically, CDMA systems use orthogonal functions for the spreading code on the forward channel and orthogonal functions for the modulation on the reverse channel.
The simplest form of a direct sequence spread spectrum system in the prior art uses coherent binary phase-shift keying (BPSK) for both the data modulation and the spreading modulation. However, the most common form of prior art direct sequence spread spectrum systems use BPSK for the data modulation and quadrature phase-shift keying (QPSK) for the spreading modulation. The Telecommunications Industry Association (TIA) IS-95 CDMA system standard uses pseudo-orthogonal functions for spreads of code on the reverse link. The IS-95 system transmits the same BPSK data on both the in-phase and the quadrature components of the forward and reverse links. One of 64 possible modulation symbols is transmitted for each group of six code symbols, where the modulation symbol is one member of the set of 64 mutually orthogonal functions that are generated using Walsh functions. Walsh functions are generated by codeword rows of special square matrices called Hadamard matrices. The Walsh functions form an ordered set of rectangular waveforms taking only two amplitudes, +1 and -1.
In prior art CDMA cellular applications the uplink, or reverse link, allows all mobile stations accessing a radio system to share the same frequency assignment. Each mobile station uses a different time shift on the PN code so that the radio system can correctly decode the information from an individual mobile station. Data on the reverse channel are convolutionally encoded and block interleaved. The encoded and interleaved data are modulated using six code symbols modulated as one of 64 modulation symbols, wherein the modulation symbol is one of 64 mutually orthogonal waveforms that are generated using Walsh functions. Following this orthogonal spreading, the reverse traffic channel and access channel are spread in quadrature using in-phase and quadrature pilot PN sequences; the spreading modulation is offset-QPSK. No pilot signal is transmitted on the reverse channel.
A third generation of CDMA currently being developed, referred to as wideband CDMA (W-CDMA), supports larger frequency bandwidths and higher data rates in order to overcome the shortcomings of CDMA. The W-CDMA supports QPSK data on the forward and reverse links to improve data throughput. In prior art W-CDMA modulators the in-phase and quadrature signals are separated after convolutional encoding. The in-phase channel adds the pilot channel. The quadrature channel linearly adds the encoded signals. Both the in-phase and quadrature channels are then modulo-2 summed with PN sequences and sent to the modulator resulting in QPSK modulation. The reverse channel may use either 9600-, 4800-, 2400-, or 1200-bps data rates for transmission.
A problem with the current CDMA system is that it employs a non-coherent reverse link. The problem with a non-coherent link in a communications system is that it has a relatively low processing gain which means that the system is bandwidth and power inefficient, resulting in a reduced level of performance relative to higher gain systems. Furthermore, the non-coherent system is unable to support the higher data rates required to support computer communications over the cellular telephone network. Making the W-CDMA system reverse link coherent would provide approximately a 3 decibel gain over the non-coherent system which would result in better performance and, consequently, reduced transmit power. Moreover, a coherent reverse link would support encoding/decoding and modulation/demodulation schemes that would allow for increased data throughput rates and increased robustness to channel noise with a cellular telephone. This would allow for the support of communications over a cellular network that require high data rates, for example computer data and video data transmission. Therefore, an objective of the new W-CDMA system is to employ a coherent reverse link. Furthermore, while meeting the objective of employing a coherent reverse link, the new W-CDMA equipment should be compatible with equipment currently used in the CDMA cellular telephone systems so as to allow maximum reuse of current equipment.